The semiconductor industry continues to strive for improvements in the speed and performance of semiconductor devices. Strained silicon technology has been shown to enhance carrier mobility in both n-channel and p-channel devices, and thus has been of interest to the semiconductor industry as a means to improve device speed and performance. Currently, strained silicon layers are used to increase electron mobility in n-channel CMOS transistors. There has been research and development activity to increase the hole mobility of p-channel CMOS transistors using strained silicon germanium layers on silicon.
FIG. 1A illustrates a known device for improved hole mobility with an n-type silicon substrate 101, a silicon germanium layer 102, a silicon capping layer 103, a gate oxide 104, a gate 105, and N+ source/drain regions 106 and 107. FIG. 1B illustrates a band structure for the device of FIG. 1A, and indicates that some carriers or holes are at the silicon-oxide interface and some are confined in the silicon germanium layer. Both the silicon germanium and the silicon capping layers will be strained if they are thin. Alternatively, the silicon germanium layer may be graded to a relaxed or unstrained layer resulting in more stress in the silicon cap layer. The crystalline silicon layer is strained by a lattice mismatch between the silicon germanium layer and the crystalline silicon layer.
More recently, strained silicon layers have been fabricated on thicker relaxed silicon germanium layers to improve the mobility of electrons in NMOS transistors. Structures with strained silicon on silicon germanium on insulators have been described as well as structures with strained silicon over a localized oxide insulator region. These structures yield high mobility and high performance transistors on a low capacitance insulating substrate.
Wafer bending has been used to investigate the effect of strain on mobility and distinguish between the effects of biaxial stress and uniaxial stress. Bonding a semiconductor onto bowed or bent substrates has been disclosed to introduce strain in the semiconductor. Stress can also be introduced by wafer bonding. Packaging can introduce mechanical stress by bending. Compressively-strained semiconductor layers have been bonded to a substrate.
FIGS. 2-4 illustrate some known techniques to strain channels and improve carrier mobilities in CMOS devices. FIG. 2 illustrates a known device design to improve electron mobility in NMOS transistors using a tensile strained silicon layer on silicon germanium. As illustrated, a graded silicon germanium layer 208 is formed on a p-type silicon substrate 209 to provide a relaxed silicon germanium region 210, upon which a strained silicon layer 211 is grown. The transistor channel is formed in the strained silicon layer 211. There is a large mismatch in the cell structure between the silicon and silicon germanium layers, which biaxially strains the silicon layer. The biaxial strain modifies the band structure and enhances carrier transport in the silicon layer. In an electron inversion layer, the subband splitting is larger in strained silicon because of the strain-induced band splitting in addition to that provided by quantum confinement. As illustrated in FIG. 3, uniaxial compressive stress can be introduced in a channel 312 of a PMOS transistor to improve hole mobility using silicon germanium source/drain regions 313 in trenches adjacent to the PMOS transistor. Large improvements in hole mobility, up to 50%, have been made in PMOS devices in silicon technology using strained silicon germanium source/drain regions to compressively strain the transistor channel. Silicon-carbon source/drain regions in trenches adjacent to an NMOS transistor can introduce tensile stress and improve electron mobility. FIG. 4 illustrates a known device design to improve mobility for both NMOS and PMOS transistors using silicon nitride capping layers 414. These silicon nitride capping layers can be formed to introduce tensile stress for NMOS transistors and can be formed to introduce compressive stress for PMOS transistors.